Abstract It is widely accepted that cell-type specific gene regulation is primarily achieved by cell-type specific expression of transcription factors (TFs), which bind to specific DNA sequence and initiate changes in higher- order chromatin structures by recruiting chromatin modifiers. While TFs are often expressed in a cell-type specific manner, chromatin modifiers are ubiquitously expressed in general. It remains largely unknown if cell- type specific chromatin modifiers govern unique chromatin structure. Recent genome-wide studies of the genetic basis of cognitive deficits such as autism and intellectual disability syndromes have implicated numerous genes involved in methylation of histones. Why is the brain so sensitive to histone methylation dysregulation? Is neuronal chromatin unique compared to other cell types? The research proposed here begins to address these major questions by testing new concept that neuron-specific splicing events govern unique methyl-histone regulation, thereby ensuring proper neuronal development. The focus in the proposed project is the LSD1-PHF21A histone demethylation complex. LSD1 is a histone demethylase, which is specific for mono- and di-methylated histone H3 lysine 4 (H3K4me1/2). PHF21A is the first-discovered unmethylated-histone ?reader? protein, which recognizes unmethylated H3K4 (H3K4me0), the reaction product of canonical LSD1 (LSD1-c). Both LSD1 and PHF21A haploinsufficiencies lead to neurodevelopmental disorders, suggesting their importance in brain development. Meanwhile, a recent study showed that 3-27 nucleotide ?microexons? are predominantly generated in the neuronal via alternative splicing. Microexons are evolutionally conserved and misregulated in autism patients and predicted to influence protein-protein interactions. Intriguingly, the neuronal LSD1 isoform (LSD1-n), which carries a microexon in its catalytic domain, appears to have distinct substrate specificity. However, it remains controversial which lysine(s) LSD1-n targets and what LSD1-n does as a multi-subunit complex within neurons. The overall objective of this proposal is to explore the neuron-specific roles of the LSD1-PHF21A complex. The research team will 1) compare the function of canonical PHF21A and neuron-specific PHF21A isoform (PHF21A-n) in histone binding and LSD1-mediated demethylation, and 2) determine the roles of PHF21A-n in synapse development and the neuronal transcriptome. The proposed project is exploratory, because analysis of the neuron-specific methyl-histone regulatory complex lacks precedents; therefore, the project fits well with the R21 mechanism. Completion of these aims will be the first test of the hypothesis that methyl histone regulatory complexes function uniquely in post-mitotic neurons as a result of neuron-specific alternative splicing. As such, this research will provide both mechanistic insight into the regulation of histone modifications and a better understanding of the pathogenesis of neurodevelopment disorders, which could lead to novel approaches for brain-specific therapeutic targets.